libgit2 has two distinct requirements that were previously solved by
`git_buf`.  We require:

1. A general purpose string class that provides a number of utility APIs
   for manipulating data (eg, concatenating, truncating, etc).
2. A structure that we can use to return strings to callers that they
   can take ownership of.

By using a single class (`git_buf`) for both of these purposes, we have
confused the API to the point that refactorings are difficult and
reasoning about correctness is also difficult.

Move the utility class `git_buf` to be called `git_str`: this represents
its general purpose, as an internal string buffer class.  The name also
is an homage to Junio Hamano ("gitstr").

The public API remains `git_buf`, and has a much smaller footprint.  It
is generally only used as an "out" param with strict requirements that
follow the documentation.  (Exceptions exist for some legacy APIs to
avoid breaking callers unnecessarily.)

Utility functions exist to convert a user-specified `git_buf` to a
`git_str` so that we can call internal functions, then converting it
back again.

When setting up relative URLs for a submodule, then we resolve it to
the actual location and write that into ".git/config" instead of
writing the relative value. We do not yet have a test to nail down this
behaviour, which is now being added by this commit.

In addition to mapping enums to strings in the configuration, we need to
know to delete the configuration option when given the "none" or "no"
option.

The current code will always fail, but only because it's asking for a
string on a live config. Take a snapshot and make sure we fail with
ENOTFOUND instead of any old error.

We no longer have any setters which affect an instance, so
`git_submodule_save()` is no longer relevant.

With this one, we can get rid of the edit_and_save test.

Similarly to the other ones. In this test we copy over testing
`RECURSE_YES` which shows an error in our handling of the `YES` variant
which we may have to port to the rest.

Moving on with the removal of runtime-changing variables, the update
setting for a remote is whatever it was when it was looked up.